In recent years, optical communication technology has come to be broadly used in a variety of fields. In optical communications, optical signals are transmitted using optical fibers as a medium. In order to switch the optical signal transmission path from one fiber to another, generally use is made of a so-called optical switching device. In achieving good optical communication, as the properties sought for optical switching and the switching operation, there is a need for optical devices having large capacity, high-speed, and high reliability. From this perspective, as optical switching devices, there is heightened expectation relating to the combining micro-mirror elements manufactured by micromachining technology. By means of a micro-mirror element, switch processing can be accomplished between the input side optical transmission path and the output side optical transmission path in an optical switching device, using an optical signal in its existent state, without converting the optical signal to an electric signal, which is desirable in obtaining the above-noted properties.
The micro-mirror element is provided with a mirror surface to reflect light, which enables changing the direction of the light reflection by oscillating the mirror surface. Micro-mirror elements of the static electricity drive type which use electrostatic power to oscillate the mirror surface is used by many optical devices. A electrostatic drive micro-mirror element can be largely classified into two types, that is, a micro-mirror element manufactured by so-called surface micromachining technology and a micro-mirror element manufactured by so-called bulk machining technology.
With surface micromachining technology, a thin film material corresponding to each of the construction members is processed onto a substrate in a desired pattern, and by successively accumulating such a pattern, formation is accomplished of each of the members constructing elements such as a support body, a mirror surface and an electrode and the like, or a subsequently removed sacrificial layer. A electrostatic drive type micro-mirror element constructed by this type of surface micromachining technology is disclosed, for example, in JP-A-H07-287177.
On the other hand, with bulk micromachining technology, by etching the material substrate itself, a support body or mirror and the like can be formed into a desired shape, and the mirror surface or electrode can be formed into a thin film, in accordance with the need. The electrostatic drive type micro-mirror element manufactured by such bulk micromachining technology is disclosed, for example, in JP-A-H09-146032, JP-A-H09-146034, JP-A-H10-62709, and JP-A-2001-13443.
As one of the technical items required in a micro-mirror element, a mirror surface should have a high degree of flatness, to ensure light reflection. By means of surface micromachining technology, since the surface of a finally formed mirror is thin, the mirror surface is easily bent, and in assuring a high degree of flatness, the size of the mirror surface need have a length of several 10 μm.
On the other hand, according to bulk micromachining technology, a mirror portion is formed by subjecting a relatively thick material substrate to etching, and then a mirror surface is provided on this mirror portion. In this manner, even with a mirror surface having a wide surface area, rigidity can be assured. As a result, it becomes possible to form a mirror surface provided with a sufficiently high degree of optical flatness. Therefore, particularly with the construction of a micro-mirror element in which the mirror surface of the length of one border must be 100 μm or greater, bulk micromachining technology is widely adopted.
FIG. 18 and FIG. 19 show a conventional static drive type micro-mirror element 400 manufactured by means of bulk micromachining technology. FIG. 18 is an exploded perspective view of micro-mirror element 400, and FIG. 19 is a cross-sectional view taken along the line XIX-XIX of FIG. 18 in a micro-mirror element 400 in the assembled state. Micro-mirror element 400 has construction in which the mirror substrate 410 and the base substrate 420 form cumulative layers. The mirror substrate 410 is formed from a set of torsion bars 413 which connect the mirror 411 with the frame 412. Relative to a specific material substrate comprised of a silicon substrate and the like which has conductive characteristics, by etching from one of its sides, formation can be accomplished of the outer contour of a mirror 411, a frame 412 and a set of torsion bars 413. To the surface of the mirror component 411 is attached a mirror surface 414. To the rear surface of the mirror 411 is attached a set of electrodes 415a and 415b. To the base substrate 420 is attached an electrode 421a which faces the electrode 415a of the mirror 411, and an electrode 421b which faces the electrode 415b. 
In the micro-mirror element 400, if an electric potential is applied to frame 412 of the mirror substrate 410, it is generally formed with the same conductive material as that of the frame 412. Electric potential is transmitted to the electrode 415a an electrode 415b through the set of torsion bars 413 and mirror 411. Furthermore, by applying a specific electric potential to the frame 412, electrification can ordinarily be accomplished, for example, of electrodes 415a and 415b. In this state, if a negative electric charge is applied to electrode 421a of the base substrate 420, then electrostatic attraction is generated between the electrode 415a and electrode 421a, and while twisting a set of torsion bars 413, the mirror 411 rotates in the direction of the arrow M1. If the mirror 411 vibrates until the sum of the twist resistance force of the torsion bars 413 and the electrostatic attraction between the electrodes reaches a point of equilibrium, then it stands still.
Instead of this, in a state in which the electrode 415a and the electrode 415b of the mirror 411 is positively charged, if a negative charge is applied to the electrode 421b, static attraction is generated between the electrode 415b and the electrode 421b, and the mirror 411 vibrates in a direction reverse of that of the arrow M1, and settles down. By means of the oscillating drive of the mirror 411, the reflection direction of the light reflected by the mirror surface 414 is switched.
In the micro-mirror element 400, however, the driving of the mirror 411, i.e., movable member, is performed by the application of only one electric potential. Specifically, if electric potential is applied to frame 412 of the mirror substrate 410, then the electric potential is transmitted to the mirror 411 through the set of torsion bars 413, and the mirror and 411 and the attached electrodes 415a and 415b come to have the same electric potential. In micro-mirror element 400, the application of different electric potential cannot be applied to the electrodes 415a and 415b to drive the movable unit. Owing to this, with the micro-mirror element 400, since the degree of freedom is low relative to the driving state of the movable unit, there is difficulty in realizing complex operations with the movable member. The conventional micro-mirror element 400 may fail to meet the requirements for an optical switching element built into an optical communications device, for example.